DRAM cell and method of fabricating the same

ABSTRACT

A method of fabricating a DRAM cell and the DRAM cell include a substrate, and a bit line formed in a first direction on the substrate. A channel region is then formed on a portion of the bit line. The channel region has a lateral surface extending vertically from the bit line. A first insulating layer is formed over the substrate, excluding the channel region, and is formed on at least a portion of the lateral surface of the channel region. A gate electrode is formed on a portion of the first insulating layer, which is on the portion of the lateral surface of the channel region, and a word line, connected to the gate electrode, is formed in a second direction on the first insulating layer. A second insulating layer is then formed over a portion of the substrate. The second insulating layer has a contact hole which exposes the channel region. Next, a capacitor is formed on a portion of the second insulating layer and on the channel region via the contact hole.

This application is a divisional of application Ser. No. 08/883,171,filed on Jun. 26, 1997, now U.S. Pat. No. 5,994,729 the entire contentsof which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a DRAM cell and, more particularly, toa DRAM cell and a method of fabricating the same, in which the DRAM cellis formed having a vertical structure in order to decrease the number offabrication steps while increasing packing density.

2. Discussion of Related Art

Generally, a DRAM cell having a horizontal structure consists of one bitline, one word line, one access transistor and one storage capacitor.The gate of the access transistor is connected to the word line, and itsdrain is connected to the bit line. Various structures of DRAM cellarrays have been proposed in order to increase the packing density ofthe DRAM.

A conventional DRAM cell will be explained below with reference toFIG. 1. FIG. 1 is a cross-sectional view of the conventional DRAM cellstructure. As shown in FIG. 1, the conventional DRAM cell is constructedin such a manner that a p-type well 11 is formed in an n-typesemiconductor substrate 10, a gate electrode 13 is formed on an activeregion of p-type well 11 and is insulated from the active region by agate oxide layer 12.

The conventional DRAM cell further includes a gate cap oxide layer 14,sidewall oxide layer 15, drain and source regions 16a and 16b formed ina portion of p-type well 11 placed on both sides of gate electrode 13,and a first interlevel insulating layer 17 formed on the overall surfaceof the substrate including gate electrode 13.

The first interlevel insulating layer 17 has a first contact holeexposing the drain region 16a. Furthermore, a bit line 19 fortransmitting data is formed on first interlevel insulating layer 17, andis electrically connected to drain region 16a via the contact hole.Second and third interlevel insulating layers 21 and 22 are formed overthe substrate 10. Second and third interlevel insulating layers 21 and22 have a second contact hole formed therein exposing source region 16b.Moreover, a capacitor storage electrode 23 is formed on third interlevelinsulating layer 22 and is electrically connected to source region 16bthrough the second contact hole. The capacitor storage electrode 23 hasa protrusion at both its edges. A dielectric layer 24 is formed on thecapacitor storage electrode 23, and a capacitor plate electrode 25 isformed on dielectric layer 24. Also, as shown in FIG. 1, sidewallinsulating layers 18a and 18b are formed on the inner sidewall of thefirst and second contact holes, respectively, and the bit line 19 has adouble structure with a conductive layer 20 laminated thereon.

In the aforementioned conventional DRAM cell, the source region 16b, thedrain region 16a, and channel region therebetween are formedhorizontally, and the gate oxide layer 12 and gate electrode 13 areformed on the channel region. The conventional DRAM cell has thefollowing problems. First, when the conventional DRAM cell structure isapplied to a highly integrated device, the short channel effect isincreased since the source region, drain region and channel region areformed horizontally. Also, the capacitance is reduced if the size of theDRAM cell is decreased.

Secondly, the bit line and capacitor are sequentially formed to contactthe source and drain regions of the transistor, respectively. This makesthe photolithography processes complicated. Also, it is difficult tosecure a margin of error during the photolithography processes.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a DRAM cell and amethod of fabricating the same that substantially obviates one or moreof the problems due to limitations and disadvantages of the related art.

An object of the present invention is to provide a vertical-structureDRAM cell suitable for a highly integrated device.

These other objects are achieved by providing a DRAM cell, comprising: asubstrate; a bit line formed in a first direction on said substrate; achannel region formed on a portion of said bit line, said channel regionhaving a lateral surface extending vertically from said bit line; afirst insulating layer formed over said substrate, excluding saidchannel region, and formed on at least a portion of said lateral surfaceof said channel region; a gate electrode formed on a portion of saidfirst insulating layer which is on said portion of said lateral surfaceof said channel region; a word line connected to said gate electrode andformed in a second direction on said first insulating layer; a secondinsulating layer formed over a portion of said substrate and having acontact hole which exposes said channel region; and a capacitor formedon a portion of said second insulating layer and on said channel regionvia said contact hole.

These and other objects are also achieved by providing a method offabricating a DRAM cell, comprising: (a) providing a substrate; (b)forming a bit line in a first direction on said substrate; (c) forming achannel region on a portion of said bit line, said channel region havinga lateral surface extending vertically from said bit line; (d) forming afirst insulating layer over said substrate, excluding said channelregion, and on at least a portion of said lateral surface of saidchannel region; (e) forming a first conductive layer in a seconddirection on said first insulating layer to serve as a word line, and ona portion of said first insulating layer, which is on said portion ofsaid lateral surface of said channel region, to serve as a gateelectrode; (f) forming a second insulating layer over a portion of saidsubstrate such that said second insulating layer covers said channelregion; (g) forming a contact hole in said second insulating layer whichexposes said channel region; and (h) forming a capacitor on a portion ofsaid second insulating layer and on said channel region via said contacthole.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of thedrawings:

In the drawings:

FIG. 1 is a cross-sectional view of a conventional DRAM cell;

FIGS. 2a and 2b are plan views of a DRAM cell array according to a firstembodiment of the present invention;

FIG. 3 is a cross-sectional view of the DRAM cell according to the firstembodiment of the present invention taken along line III--III of FIG.2b;

FIG. 4 is a cross-sectional view of the DRAM cell according to the firstembodiment of the present invention taken along line IV--IV of FIG. 2b;

FIGS. 5a to 5j are cross-sectional views corresponding to FIG. 3 showinga process of fabricating the DRAM cell according to the first embodimentof the present invention;

FIG. 6 is a cross-sectional view corresponding to FIG. 3 of a DRAM cellaccording to a second embodiment of the present invention;

FIG. 7 is a cross-sectional view corresponding to FIG. 4 of the DRAMcell according to the second embodiment of the present invention; and

FIGS. 8a to 8i are cross-sectional views corresponding to FIG. 6 showinga process of fabricating the DRAM cell according to the secondembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

FIGS. 2a and 2b are plan views showing a DRAM cell according to a firstembodiment of the present invention. FIG. 2a shows a gate electrode 36aand channel region 34 of the DRAM cell of the present invention.Referring to FIG. 2a, a word line 36 and bit line 32 intersect eachother, and a channel region 34 of the DRAM cell is formed at theintersection. A gate oxide layer 35 surrounds channel region 34, and thegate electrode 36a surrounds the channel region 34 and the gate oxidelayer 35.

Referring to FIG. 2b, word line 36 and bit line 32 are arrangedperpendicular to each other, and gate electrode 36a is formed at theportion where the word line 36 and bit line 32 intersect each other.Gate electrode 36a is formed in the shape of a ring. A first conductivelayer 38 surrounds gate electrode 36a, but is separated therefrom by afirst insulating layer 37 (see FIG. 3). A first dielectric layer 39(shown in FIG. 3) surrounds first conductive layer 38, and a secondconductive layer 41 surrounds the first dielectric layer 39.Furthermore, a second dielectric layer 43 (shown in FIG. 3) surroundssecond conductive layer 41, and a third conductive layer 44 surroundsthe second dielectric layer 43. As described above, bit line 32, gateelectrode 36a and a capacitor region are sequentially formed.Accordingly, it is possible to realize a high-integrated DRAM easily.

The DRAM cell structure according to the first embodiment of the presentinvention will be explained below with reference to FIGS. 3 and 4. FIG.3 is a cross-sectional view of the DRAM cell according to the firstembodiment of the present invention taken along line III--III of FIG.2b, and FIG. 4 is a cross-sectional view of the DRAM cell according tothe first embodiment of the present invention taken along line IV--IV ofFIG. 2b.

The DRAM cell is constructed in such a manner that a p-typesemiconductor layer 31 is formed in a predetermined portion of an n-typesubstrate 30 in order to insulate bit line 32 from substrate 30, and bitline 32 is formed in one direction in p-type semiconductor layer 31. Bitline 32 is formed in such a manner that an n-type impurity is injectedinto p-type semiconductor layer 31.

Furthermore, a vertical channel region 34 having a cylindrical shape isformed on a predetermined portion of bit line 32, and a first gate oxidelayer 33 is formed over the portion of substrate 30 not covered by thechannel region 34. A gate oxide layer 35 surrounds channel region 34,and is formed higher than channel region 34. A vertical gate electrode36a is formed along the sides of gate oxide layer 35. The height of thegate electrode 36a is less than the height of gate oxide layer 35.

A first insulating layer 37 and first conductive layer 38 aresequentially formed to surround gate electrode 36a. A first dielectriclayer 39 is formed on first conductive layer 38. Neither the firstinsulating layer 37, the first conductive layer 38, nor the firstdielectric layer 39, however, are formed on the channel region 34. Asecond conductive layer 41 is then formed over the first dielectriclayer 39 and the channel region 34, and thus, comes into contact withchannel region 34 to serve as a capacitor storage node surrounding firstdielectric layer 39. A second dielectric layer 43 is formed on andsurrounds second conductive layer 41. The second dielectric layer 43 isalso formed on the first oxide layer 33 and word line 36. A thirdconductive layer 44, serving as a capacitor plate node, is formed onsecond dielectric layer 43, and a second insulating layer 45 is formedon the overall surface of third conductive layer 44.

As described above, the capacitor of the DRAM cell is constructed insuch a manner that first conductive layer 38 is connected to secondconductive layer 41 used as the capacitor storage node. This increasesthe resulting capacitance. Furthermore, since bit line 32, gateelectrode 36a and the capacitor are sequentially laminated and channelregion 34 is formed vertically, the integration and operation speed ofthe DRAM are increased.

A method of fabricating the DRAM cell according to the first embodimentof the present invention will be explained below with reference to FIGS.5a to 5j. FIGS. 5a-5j are cross-sectional views corresponding to FIG. 3.Referring to FIG. 5a, a p-type impurity such as boron is ion-implantedinto an n-type substrate 30 to a predetermined depth; thereby forming ap-type semiconductor layer 31. Then, an n-type impurity such asphosphorus is ion-implanted in higher concentration into the p-typesemiconductor layer 31 to form a bit line 32 in p-type semiconductorlayer 31. The bit line 32 serves as the source of the transistor.

Referring to FIG. 5b, a first oxide layer 33 is formed on the bit line32, and then a predetermined photoresist pattern (not shown) is formedthereon. First oxide layer 33 is selectively etched, using thephotoresist pattern as a mask, to thereby form a hole. Thereafter, thephotoresist is removed. Referring to FIG. 5c, a silicon layer is formedin the hole through selective epitaxy to thereby form a channel region34 having a cylindrical shape in the hole. First oxide layer 33 is thenetched through isotropic etching to a predetermined thickness on the bitline 32.

Referring to FIG. 5d, an oxide layer is formed over the overall surfaceof the substrate 30 and anisotropic-etched to thereby form gate oxidelayer 35 on the sides of channel region 34. Referring to FIG. 5e, apolysilicon layer 36' is formed over the overall surface of thesubstrate 30. The polysilicon layer 36' will be used as word line 36 andgate electrode 36a. Referring to FIG. 5f, the polysilicon layer 36' isanisotropic-etched to surround the side of gate oxide layer 35 therebyforming gate electrode 36a and word line 36. Here, the polysilicon layer36' is over-etched. By doing so, the resulting height of both channelregion 34 and gate electrode 36a is less than the height of gate oxidelayer 35.

Referring to FIG. 5g, a first insulating layer 37, first conductivelayer 38 and first dielectric layer 39 are sequentially formed over theoverall surface of the substrate 30. Preferably, the first conductivelayer 38 is formed of the same material as the bit line 32. Referring toFIG. 5h, a photoresist layer is formed on first dielectric layer 39, andselectively exposed and developed through photolithography to therebyform a first photoresist pattern 40. Then, a portion of first dielectriclayer 39, first conductive layer 38 and first insulating layer 37 overchannel region 34 are sequentially etched using first photoresistpattern 40 as a mask. Thereafter, first photoresist pattern 40 isremoved.

Referring to FIG. 5i, a second conductive layer 41 is formed on theexposed portion of channel region 34 and first dielectric layer 39.Preferably, the second conductive layer 41 is formed of the samematerial as the bit line 32. Then a second photoresist pattern 42 isformed on a predetermined portion of second conductive layer 41. Secondconductive layer 41 forms both the drain of the transistor and thecapacitor storage node. Also preferable is forming second conductivelayer 41 of a polysilicon layer with an n-type impurity, such asphosphorus, doped thereinto. Thereafter, second conductive layer 41,first dielectric layer 39, first conductive layer 38, and firstinsulating layer 37 are sequentially etched using second photoresistpattern 42 as a mask.

Referring to FIG. 5j, after removing second photoresist pattern 42, asecond dielectric layer 43 and third conductive layer 44, forming acapacitor plate node, are sequentially formed over a portion of theoverall surface of the substrate 30. Preferably, the third conductivelayer is formed of the same material as the channel region 34. A secondinsulating layer 45 is then formed thereon to complete formation of theDRAM cell.

The DRAM cell according to the second embodiment of the presentinvention will be explained below with reference to FIGS. 6 and 7. FIGS.6 and 7 show cross-sectional views corresponding to FIGS. 3 and 4,respectively. The DRAM cell of the second embodiment is constructed insuch a manner that an insulating oxide layer 61 is formed on aninsulating substrate 60, and a silicon layer 62 is formed in onedirection on oxide layer 61; thereby forming a silicon on insulator(SOI) structure. A vertical channel region 64 having a cylindrical shapeis formed on a predetermined portion of the SOI, and a first oxide layer63 is formed over the portion of substrate 60 not covered by the channelregion 64. A gate oxide layer 65 surrounds channel region 64, and isformed higher than channel region 64.

A vertical gate electrode 66a surrounds gate oxide layer 65. Gateelectrode 66a has a height less than the height of gate oxide layer 65.A word line 66 is formed on first oxide layer 63, and is connected togate electrode 66a. A first insulating layer 67 is formed to surroundgate electrode 66a. A first conductive layer 68 surrounds firstinsulating layer 67, and a first dielectric layer 69 surrounds firstconductive layer 68. A second conductive layer 71 surrounds firstdielectric layer 69 and is formed on the channel region 64. A seconddielectric layer 73 is formed on second conductive layer 71, first oxidelayer 63 and word line 66; third conductive layer 74 is formed on seconddielectric layer 73; and a second insulating layer 75 is formed on thirdconductive layer 74.

A method of fabricating the DRAM cell according to the second embodimentof the present invention will be explained below with reference to FIGS.8a to 8i. FIGS. 8a-8i show cross-sectional views corresponding to FIG.6. Referring to FIG. 8a, a first oxide layer 63 is formed on asemiconductor layer 62 of a silicon on insulator (SOI) structure. TheSOI structure is formed of insulating substrate 60, insulating oxidelayer 61 and silicon layer 62. Here, silicon layer 62 is used as a bitline. A predetermined photoresist pattern (not shown) is formed on firstoxide layer 63. First oxide layer 63 is selectively etched using thephotoresist pattern as a mask to thereby form a hole. Thereafter, thephotoresist is removed.

Referring to FIG. 8b, a silicon layer is formed in the hole throughselective epitaxy to form a channel region 64 having a cylindrical shapein the hole. First oxide layer 63 is then etched through isotropicetching to a predetermined thickness.

Referring to FIG. 8c, an oxide layer is formed on the overall surface ofthe substrate and anisotropic-etched to thereby form a gate oxide layer65 surrounding channel region 64. Referring to FIG. 8d, a polysiliconlayer 66' is formed over the overall surface of the substrate 60. Thepolysilicon layer 66' will be used as word line 66 and gate electrode66a. Referring to FIG. 8e, the polysilicon layer 66' isanisotropic-etched to surround the sides of gate oxide layer 65; therebyforming the gate electrode 66a and word line 66. Here, the polysiliconlayer 66' is over-etched. By doing so, the resulting height of bothchannel region 64 and gate electrode 66a is less than the height of gateoxide layer 65.

Referring to FIG. 8f, a first insulating layer 67, first conductivelayer 68 and first dielectric layer 69 are sequentially formed over theoverall surface of the substrate 60. Preferably, the first conductivelayer 68 is formed of the same material as bit line 62. Referring toFIG. 8g, a photoresist layer is formed on first dielectric layer 69, andselectively exposed and developed through photolithography to therebyform a first photoresist pattern 70. Then, a portion of first dielectriclayer 69, first conductive layer 68 and first insulating layer 67 overchannel region 64 are sequentially etched using first photoresistpattern 70 as a mask.

Referring to FIG. 8h, after removing first photoresist pattern 70, asecond conductive layer 71 is formed on the exposed portion of channelregion 64 and first dielectric layer 69. Then a second photoresistpattern 72 is formed on a predetermined portion of second conductivelayer 71. Second conductive layer 71 forms both the drain of thetransistor and the capacitor storage node. Preferably, second conductivelayer 71 is formed of the same silicon material forming the bit line 62.

Thereafter, second conductive layer 71, first dielectric layer 69, firstconductive layer 68, and first insulating layer 67 are sequentiallyetched using second photoresist pattern 72 as a mask. Then, secondphotoresist pattern 72 is removed. Referring to FIG. 8i, a seconddielectric layer 73, and third conductive layer 74, forming a capacitorplate node, are sequentially formed over a portion of the overallsurface of the substrate 60, and a second insulating layer 75 is formedon the third conductive layer 74 to complete the formation of the DRAMcell. Preferably, the third conductive layer 74 is formed of the samematerial as the channel region 64.

When the aforementioned SOI structure is used, silicon layer 62, formingthe source and bit line, is securely isolated from substrate 60 by oxidelayer 61, thereby improving the performance of the DRAM.

The present invention has the following advantages. First, since the bitline, gate electrode and capacitor are sequentially formed, a highlyintegrated device can be obtained. Secondly, the channel region has acylindrical structure so that its area is enlarged, increasing theoperation speed of the device. Thirdly, the capacitor fabricationprocess is simplified, and well formation, isolation, ion implantation,heat treatment and diffusion processes can be omitted. Accordingly, thenumber of fabrication steps is reduced, and thus productivity isincreased. This results in a lower device price.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the DRAM cell and method offabricating the same of the present invention without departing from thespirit or scope of the invention. Thus, it is intended that the presentinvention cover the modifications and variations of this invention.

What is claimed is:
 1. A method of fabricating a DRAM cell,comprising:(a) providing a substrate; (b) forming a bit line in a firstdirection on said substrate; (c) forming a channel region on a portionof said bit line, said channel region having a lateral surface extendingvertically from said bit line; (d) forming a first insulating layer oversaid substrate, excluding said channel region, and on at least a portionof said lateral surface of said channel region; (e) forming a firstconductive layer in a second direction on said first insulating layer toserve as a word line, and on a portion of said first insulating layer,which is on said portion of said lateral surface of said channel region,to serve as a gate electrode; (f) forming a second insulating layer overa portion of said substrate such that said second insulating layercovers said channel region; (g) forming a contact hole in said secondinsulating layer which exposes said channel region; and (h) forming acapacitor on a portion of said second insulating layer and on saidchannel region via said contact hole.
 2. The method of claim 1, whereinsaid first direction and said second direction are perpendicular.
 3. Themethod of claim 1, wherein said step (d) forms said first insulatinglayer on and surrounding said lateral surface of said channel region. 4.The method of claim 3, wherein said step (e) forms said first conductivelayer such that said gate electrode surrounds said first insulatinglayer formed on and surrounding said lateral surface of said channelregion.
 5. The method of claim 1, wherein said step (c) forms saidchannel region having a vertical cylindrical shape.
 6. The method ofclaim 1, wherein said channel region has a first height, said firstinsulating layer formed on said portion of said lateral surface of saidchannel region has a second height, said gate electrode has a thirdheight, and said second height is greater than both said first and thirdheights.
 7. The method of claim 1, wherein said step (h) comprises:(h1)forming a storage electrode on said portion of said second insulatinglayer and electrically connected to said channel region via said firstcontact hole; (h2) forming a first dielectric layer on said storageelectrode; and (h3) forming a plate electrode on said first dielectriclayer.
 8. The method of claim 7, wherein said step (h1) comprises:(h11)forming a second conductive layer on said portion of said secondinsulating layer; (h12) forming a second dielectric layer on said secondconductive layer; and (h13) forming a third conductive layer on saidsecond dielectric layer and on said channel region via said firstcontact hole.
 9. The method of claim 8, whereinsaid step (a) includesthe steps of (a1) providing a base substrate of a first conductivitytype, and (a2) forming a semiconductor layer of a second conductivitytype on said base substrate; and said step (b) forms an impurity regionof said first conductivity type in said semiconductor layer to form saidbit line.
 10. The method of claim 9, whereinsaid step (h11) forms saidsecond conductive layer from the same material as said bit line; andsaid step (h13) forms said third conductive layer from the same materialas said bit line.
 11. The method of claim 7, wherein said step (h3)forms said plate electrode from the same material used to from saidchannel region.
 12. The method of claim 1, whereinsaid step (a) includesthe steps of (a1) providing a base substrate of a first conductivitytype, and (a2) forming a semiconductor layer of a second conductivitytype on said base substrate; and said step (b) forms an impurity regionof said first conductivity type in said semiconductor layer to form saidbit line.
 13. The method of claim 1, whereinsaid step (a) includes thesteps of (a1) providing an insulating base substrate, and (a2) formingan insulating oxide layer on said insulating base substrate; and saidstep (b) forms a silicon layer in said insulating oxide layer to formsaid bit line.
 14. The method of claim 1, wherein said step (c) formssaid channel region through selective epitaxy.
 15. The method of claim1, wherein said step (h) forms said capacitor to have, when viewedvertically, a circular shape centered on said channel region.